Method and apparatus for the reliquefaction of a vapour

ABSTRACT

Boiled off liquefied natural gas flows from a storage tank and is compressed in vapour compression stages. The resulting compressed vapour is condensed in a condenser and the condensate returned to the tank. The condenser is cooled by a working fluid, for example nitrogen, flowing in a Brayton cycle. The Brayton cycle includes a heat exchanger which removes heat of compression from the compressed natural gas vapour upstream of its passage through the condenser. In addition a part of the working fluid is withdrawn from a region of the Brayton cycle intermediate the working fluid outlet from the condenser and the working fluid inlet to the heat exchanger and the withdrawn working fluid flows through another heat exchanger in which it removes heat of compression from the natural gas vapour intermediate the compression stages. The withdrawn working fluid is returned to the Brayton cycle.

This invention relates to a method and apparatus for the reliqueficationof a vapour, particularly a method and apparatus which are operable onboard ship to reliquefy natural gas vapour.

Natural gas is conventionally transported over large distances inliquefied state. For example, ocean going tankers are used to conveyliquefied natural gas from a first location in which the natural gas isliquefied to a second location in which it is vaporised and sent to agas distribution system. Since natural gas liquefies at cryogenictemperatures, i.e.. temperatures below −100° C., there will becontinuous boil-off of the liquefied natural gas in any practicalstorage system. Accordingly, apparatus needs to be provided in order toreliquefy the boiled-off vapour. In such an apparatus a refrigerationcycle is performed comprising compressing a working fluid in a pluralityof compressors, cooling the compressed working fluid by indirect heatexchange, expanding the working fluid, and warming the expanded workingfluid in indirect heat exchange with the compressed working fluid, andreturning the warmed working fluid to one of the compressors. Thenatural gas vapour, downstream of a compression stage, is at leastpartially condensed by indirect heat exchange with the working fluidbeing warmed. One example of an apparatus for performing such arefrigerant method is disclosed in U.S. Pat. No. 3,857,245.

According to U.S. Pat. No. 3,857,245 the working fluid is derived fromthe natural gas itself and therefore an open refrigeration cycle isoperated. The expansion of the working fluid is performed by a valve.Partially condensed natural gas is obtained. The partially condensednatural gas is separated into a liquid phase which is returned tostorage and a vapour phase which is mixed with natural gas being sent toa burner for combustion. The working fluid is both warmed and cooled inthe same heat exchanger so that only one heat exchanger is required. Theheat exchanger is located on a first skid-mounted platform and theworking fluid compressors on a second skid-mounted platform.

Nowadays, it is preferred to employ a non-combustible gas as the workingfluid. Further, in order to reduce the work of compression that needs tobe supplied externally, it is preferred to employ an expansion turbinerather than a valve in order to expand the working fluid.

An example of an apparatus which embodies both these improvements isgiven in WO-A-98/43029. Now two heat exchangers are used, one to warmthe working fluid in heat exchange with the compressed natural gasvapour to be partially condensed, and the other to cool the compressedworking fluid.

WO-A-98/43029 points out that incomplete condensation of the natural gasvapour reduces the power consumed in the refrigeration cycle (incomparison with complete condensation) and suggests that the residualvapour—which is relatively rich in nitrogen—should be vented to theatmosphere. Indeed, the partial condensation disclosed in WO-A-98/43029follows well known thermodynamic principles which dictate that thecondensate yield is purely a function of the pressure and temperature atwhich the condensation occurs.

Typically, the liquefied natural gas may be stored at a pressure alittle above atmospheric pressure and the boil-off vapour may bepartially condensed at a pressure of 4 bar. The resulting partiallycondensed mixture is typically flashed through an expansion valve into aphase separator to enable the vapour to be vented at atmosphericpressure. Even if the liquid phase entering the expansion valve containsas much as 10 mole percent of nitrogen at 4 bar, the resulting vapourphase at 1 bar still contains in the order of 50% by volume of methane.In consequence, in a typical operation, some 3000 to 5000 kg of methanemay need to be vented daily from the phase separator. Since methane isrecognised as a greenhouse gas such a practice would be environmentallyunacceptable.

Another problem associated with the operation of the apparatus accordingto WO-A-98/43029, is that there are considerable thermodynamicinefficiencies caused by a mismatch between, on the one hand, thetemperature and enthalpy of the compressed natural gas and, on the otherhand, the temperature and enthalpy of the working fluid.

EP-A-1 132 698 discloses a method that mitigates the problems that arecaused when vapour is returned with condensed natural gas to a liquefiednatural gas (LNG) storage tank.

In the method according to EP-A-1 132 698 the boiled off vapour and/orthe natural gas condensate are mixed with liquefied natural gas takenfrom storage.

Since the nitrogen mole fraction in the liquefied natural gas is lessthan the nitrogen mole fraction in the boiled-off vapour and even lessthan that in flash gas formed by the expansion through the valve of thecondensed boil-off vapour, dilution of the boiled-off vapour with theliquefied natural gas either upstream or downstream of the condenser, orboth, tends to dampen swings in the composition of the vapour phase inthe storage tank that would otherwise occur without the mixing of theboiled off vapour or natural gas condensate with the liquefied naturalgas from storage.

The method according to EP-A-1 132 698 does not however greatly enhancethe overall thermodynamic efficiency.

SUMMARY OF THE INVENTION

According to the present invention there is provided a method ofreliquefying vapour boiled off from at least one volume of liquefiednatural gas held in at least one storage tank, comprising compressingthe vapour in first and second vapour compression stages in series,condensing the compressed vapour in a condenser by heat exchange with aworking fluid flowing in a main endless working fluid cycle, andreturning at least some of the resulting condensate to the said storagetank, wherein in the main working fluid cycle the working fluid is, insequence, compressed in at least one working fluid compressor, cooled ina first heat exchanger, expanded in an expansion turbine, employed inthe condenser to perform the condensation of the natural gas vapour,warmed in the said first heat exchanger in heat exchange with theworking fluid being cooled and returned to the said working fluidcompressor, characterised in that in the main working fluid cycleintermediate the passage of the working fluid through the condenser andits passage through the first heat exchanger, the working fluid isemployed to precool in a second heat exchanger the compressed naturalgas vapour downstream of the second vapour compression stage butupstream of the condenser, and in that a flow of working fluid isdiverted from a region of the main working fluid cycle where the workingfluid is flowing from the condenser to the second heat exchanger and ispassed through at least one third heat exchanger so as to cool thenatural gas vapour intermediate the first and second vapour compressionstages, the diverted working fluid being returned to the main workingfluid cycle at a region where the working fluid is flowing from thesecond heat exchanger to the first heat exchanger.

The invention also provides apparatus for reliquefying natural gasvapour comprising at least one storage tank for holding at least onevolume of liquefied natural gas, first and second vapour compressionstages in series for compressing boiled-off natural gas vapourcommunicating with at least one vapour space in the said storage tank, acondenser for condensing the compressed vapour having a natural gasinlet communicating with the second vapour compression stage and anoutlet communicating with the said storage tank, wherein the condenseris arranged so as to be cooled, in use, by a working fluid, thecondenser forming part of an endless main working fluid cyclecomprising, in sequence, (a) at least one working fluid compressor forcompressing a flow of the working fluid, (b) a cooling path through afirst heat exchanger for cooling the working fluid flow, (c) anexpansion turbine for expanding the flow of working fluid, (d) thecondenser, (e) a warming path through the first heat exchanger forwarming the working fluid, and (f) an inlet to the said working fluidcompressor, characterised in that the main working fluid cycle comprisesa second heat exchanger for cooling the natural gas by heat exchangewith the working fluid, the second heat exchanger having a natural gasvapour path therethrough intermediate the second vapour compressionstage and the condenser and a working fluid path therethroughintermediate the working fluid outlet from the condenser and the inletto the warming path through the first heat exchanger, and in that thereis a third heat exchanger for cooling the natural gas vapourintermediate the first and second natural gas vapour compression stagesby heat exchange with working fluid diverted from the main working fluidcycle, the third heat exchanger having a working fluid path therethroughcommunicating at its inlet with a region of the working fluid cycleintermediate the working fluid outlet from the condenser and the workingfluid inlet to the second heat exchanger and at its outlet with a regionof the working fluid cycle intermediate the working fluid outlet fromthe second heat exchanger and the inlet to the warming path through thefirst heat exchanger.

The method and apparatus according to the invention are able to achieveimproved thermodynamic efficiency of operation in comparison with thecorresponding methods and apparatuses disclosed in the prior documentsmentioned above. We attribute the improved thermodynamic efficiency tothe integration of the working fluid cycle and the natural gascondensation not only in the condenser but also in the second and thirdheat exchangers. The improvement in thermodynamic efficiency can beexploited by means of a reduced power consumption.

Preferably the proportion of the working fluid that is diverted from themain working fluid cycle to the third heat exchanger is controlled inresponse to the temperature at the inlet to the second vapourcompression stage.

Preferably when the said storage tank is fully laden with liquefiednatural gas, the condenser is operated such that sub-cooled liquefiednatural gas exits from it. Sometimes, however, when the said storagetank contains only a relatively small amount of liquefied natural gasreturn of the condensate to the tank has the effect of enriching theboiled-off vapour in nitrogen. In consequence the vapour presented tothe condenser for condensation may contain an excess of nitrogen withthe consequence that not only is the condensate not sub-cooled but it isnot even fully condensed. In such circumstances, or if the storage tankcontains a liquefied natural gas having a high nitrogen content, forexample, one that gives a boil-off gas containing 20 to 40% by volume ofnitrogen, the condensate, which contains uncondensed vapour, is flashedinto a phase separator, the resultant liquid phase being returned to thestorage tank and the resultant vapour phase being sent to the ship'sengines (in the case of shipboard use if the engines are powered bynatural gas) or is burnt and vented to the atmosphere.

The first and second vapour compression stages are preferably driven bya single plural speed motor.

Preferably the vapour upstream of the first vapour compression stage isprecooled by having mixed therewith a stream of condensed natural gastaken from the condenser. Preferably the flow rate of the stream ofcondensed natural gas vapour is controlled in response to thetemperature at the inlet to the first compression stage.

BRIEF DESCRIPTION OF THE DRAWING

The method and apparatus according to the invention will now bedescribed by way of example with reference to the accompanying drawing,in which:

The drawing FIGURE is a schematic flow diagram of a shipboardinstallation for the storage of liquefied natural gas (LNG).

The drawing is not to scale.

DESCRIPTION OF THE INVENTION

Referring to the drawing, the five thermally-insulated storage tanks 2,4, 6, 8 and 10 are provided in the hull of a ship or other sea-goingvessel (not shown). Two or more of the storage tanks 2, 4, 6, 8 and 10are provided with a submerged orifice pipe 12 located in its bottomregion through which LNG is introduced. For reasons of ease ofillustration, the orifice pipes in the tanks 2, 4 and 6 are not shown inthe drawing. If only some of the storage tanks are provided withsubmerged orifice pipes, redistribution of returning LNG to tanks not soprovided is by operation of liquid pumps (not shown). The orifice pipe12 is in normal operation submerged in a volume 16 of LNG. In each ofthe tanks 2, 4, 6, 8 and 10 there is a vapour space 18 above the volume16 of LNG therein.

Although the storage tanks 2, 4, 6, 8 and 10 are thermally insulated,because LNG has a boiling point at normal pressures substantially belowambient temperature there is a continuous evaporation of the LNG in eachof the storage tanks 2, 4, 6, 8 and 10. Each of the tanks has a topoutlet 22 for vapour which communicates with a boiled-off gas header 24.Extending from the header 24 is a main pipeline 26 for the boiled-offgas. Located in the pipeline 26 is a mixer 28, in which, in operation,the vapour may be mixed with condensed LNG from a downstream part of theinstallation. In operation, the condensed LNG evaporates in theboiled-off gas and thereby reduces the temperature of this gas. A sensor27 is provided downstream of the mixer and generates signalsrepresentative of the temperature at the inlet to a first compressionstage 40, which signals are relayed to a valve controller 30, which inturn controls the setting of the flow-control valve 32 in a LNGcondensate pipeline 34 that terminates in a spray nozzle 36 within themixer 28. The mixer 28 may thus be operated so as to provide natural gasat a chosen essentially constant cryogenic temperature below, say, minus100° C. to the first compression stage 40.

The boiled-off gas flows from the mixer 28 into the first compressionstage. The outlet of the first compression stage 40 indirectlycommunicates with the inlet of a second compression stage 42. Thecompression stages 40 and 42 are typically driven by a single electricmotor 44 through, if desired, an integral gearbox 45.

The motor 44 is typically able to be operated at two different speeds.

Resulting compressed gas is supplied from the second compression stage42 to a condenser 46, typically in the form of a plate fin or spirallywound heat exchanger, in which it is condensed and once condensedsubjected to sub-cooling. The resulting sub-cooled condensate flows fromthe condenser 46 along a pipeline 48 to a condensate return header 50which feeds the orifice pipes 12 in the bottom regions of the tanks 8and 10, or if each tank is equipped with the orifice pipe 12, to thetanks 2, 4, 6, 8 and 10.

Cooling for the condenser 46 is provided by a working or heat exchangefluid such as nitrogen flowing at a first pressure in an essentiallyclosed refrigeration cycle 60 such as a Brayton cycle.

In the Brayton cycle 60 nitrogen passing out of the condenser 46 iswarmed in heat exchange with returning compressed nitrogen at a secondpressure higher than the first in a gas-to-gas heat exchanger 62. Theresulting warmed nitrogen flows to a compressor 64 which typicallycomprises three compression stages 66, 68 and 70 all having rotors (notshown) mounted on an integral gearbox (not shown) or on the same shaft72 able to be driven by a motor 74 through a gearbox 75. A firstintercooler 78 is located downstream of the outlet from the firstcompression stage 66 and upstream of the inlet to the second compressionstage 68. A second intercooler 80 is located downstream of the outletfrom the second compression stage 68 and upstream of the inlet to thethird compression stage 70. An aftercooler 82 is located downstream ofthe outlet from the third compression stage 70. The intercoolers 78 and80 and the aftercooler 82 are typically all cooled by water and areoperated so as to remove the heat of compression from the circulatingnitrogen in operation of the Brayton cycle. The resulting aftercooledcompressed nitrogen flow passes through the heat exchanger 62 as thepreviously mentioned returning cold nitrogen stream. The compressednitrogen stream is thus cooled to a lower temperature in the heatexchanger 62. The compressed cooled nitrogen flow passes to an expansionturbine 84 where it is expanded with the performance of extra work. Theexpansion turbine 84 is typically mounted on the same integral gearbox(not shown) or on the same shaft as the compression stages 66, 68 and70. The expansion turbine 84 thus helps to drive the compression stages66, 68 and 70. The expansion of the nitrogen in the turbine 84 generatesthe refrigeration necessary for the condensation of the natural gasvapour in the condenser 46. The nitrogen thus continuously passesthrough an endless circuit.

A particular feature of the Brayton cycle 60 illustrated in the drawingis that the nitrogen does not pass directly from the condenser 46 to theheat exchanger 62. Instead it passes through a second gas-to-gascountercurrent heat exchanger 86. The purpose of this heat exchanger isto pre-cool the natural gas to a temperature close to its condensationtemperature upstream of entering the condenser 46. During typicaloperating conditions when the tanks 2, 4, 6, 8 and 10 are fully ladenwith LNG the natural gas is consequently not only liquefied but alsosub-cooled in the condenser 46. The sub-cooling of the liquefied naturalgas keeps down the formation of flash gas when the LNG is returned tothe tanks.

A further feature of the particular form of Brayton cycle 60 shown inthe drawing is that a part of the nitrogen is withdrawn from a region ofthe Brayton cycle downstream of the outlet from the condenser 46 butupstream of the inlet to the second heat exchanger 86 and flows througha third heat exchanger 88 which is located downstream of the firstnatural gas compression stage 40 but upstream of the second natural gascompression stage 42 and thus serves to remove the heat of compressiongenerated in the natural gas by operation of the first compression stage40. As a result, the nitrogen passing through the third heat exchanger88 is warmed. The warmed nitrogen flow is returned to the Brayton cycle60 at a region downstream of the outlet from the second heat exchanger86 but upstream of the inlet to the warming passages through the firstheat exchanger 62. Typically, a control valve 90 controls the rate offlow of nitrogen working fluid through the third heat exchanger inresponse to a temperature sensor (not shown) at the inlet to the secondnatural gas compression stage 42. In a typical arrangement, the controlvalve 90 operates to maintain a constant temperature at the inlet to thesecond natural gas compression stage 42.

Not all the natural gas that is liquefied in the condenser 46 istypically returned via the pipeline 50 to the tanks 2, 4, 6, 8 and 10. Aportion of the condensate is sent via the pipeline 34 to the mixer 28 soas to pre-cool the natural gas upstream of the first compression stage40.

In operation, there are various ways of operating the apparatus shown inthe drawing according to how laden with LNG the tanks 2, 4, 6, 8 and 10are. When these tanks are fully laden, the temperature at the inlet tothe first natural gas compression stage 40 is typically in the order ofminus 100° C. or even lower. The pressure at the inlet is typically alittle above 1 bar. The natural gas typically leaves the firstcompression stage at a temperature of minus 65° C. and a pressure in theorder of 2 bar. The gas is typically cooled in the heat exchanger to atemperature in the order of minus 130° C. and enters the second naturalgas compression stage at this temperature. The natural gas typicallyleaves the second compression stage 42 at a pressure in the order of 5bar and a temperature of about minus 75° C. The natural gas is cooled inthe second heat exchanger to a temperature at which it will begin tocondense. The exact value of this temperature will depend on thecomposition of the natural gas. The greater the mole fraction ofnitrogen in the natural gas, the lower will be the temperature at whichit starts to condense. Because the condenser 46 is not required todesuperheat the natural gas in normal operation, more efficient heatexchange is made possible than in previously known cycles in which thecorresponding condenser has been required both to desuperheat and tocondense the natural gas. As a result of the intercooling,desuperheating and separate condensing with subcooling, the powerconsumption of the refrigeration cycle is reduced.

As previously stated, the natural gas leaves the condenser 46 as asub-cooled liquid. Typically, its exit temperature is in the order ofminus 165° C. depending on the composition of the natural gas. One ofthe advantages of such a low exit temperature is that relatively little,if any, flash gas is formed on reintroduction of the LNG into the tanks2, 4, 6, 8 and 10 through the orifice pipes 12. Moreover, when the tanksare fully laden, any flash gas that is formed may be dissolved orcondensed in the liquid before it reaches the surface.

During normal operation when the tanks are fully laden the expansionturbine 84 typically has an inlet temperature in the order of minus 104°C., an outlet temperature in the order of minus 168° C., and an outletpressure in the order of 10 bar. If the composition of the natural gasis, say, 8.5% by volume of nitrogen and 91.5% by volume of methane, thistemperature is sufficiently low for the condensate produced in thecondenser 46 to have a desired degree of sub-cooling. Sometimes,however, the ship in which the tanks 2, 4, 6, 8 and 10 are located isrequired to transport sufficiently less than the maximum amount of LNGfor the liquid head in the tanks not to be sufficient to preventflashing of condensate returned through the orifice pipes 12 or toensure complete dissolution of fine bubbles of flash gas that are formedin the volumes 16 of LNG. As a result, the vapour that flows from thetanks 2, 4, 6, 8 and 10 to the first compression stage 40 is enriched innitrogen. As a consequence, its condensation temperature at the outletpressure of the second natural gas vapour compression stage 42 falls.Indeed, when the tanks are relatively lightly laden with LNG the degreeof enrichment may become so great that the condenser 46 no longer fullycondenses the vapour. In this case, instead of being passed to theconduit 50, the mixture of condensate and uncondensed vapour may beselectively directed through a valve 100 into a phase separator 102.Liquid is withdrawn from the bottom of the phase separator 102 and sentto the conduit 50. Vapour passes from the phase separator 102 to a ventline 104 which leads through a heater 106 to a gas combustion unit 108so that the natural gas content of the vapour may be burned and theresulting combustion gases vented to the atmosphere.

The minimum and maximum flows of natural gas vapour in operation of theapparatus shown in the drawing can vary widely. It is thereforetypically preferred to employ two sets of first and second natural gascompression stages 40 and 42, the two sets being in parallel with oneanother. Thus, there are typically two third heat exchangers 88 inparallel with one another. Whether one or both sets are used depends onthe rate of vaporisation of the natural gas in the tanks 2, 4, 6, 8 and10. Similarly, they may be two or more sets of nitrogen compressionstages 66, 68 and 70 in parallel, and two or more expansion turbines 84in parallel.

1. A method of reliquefying vapour boiled off from at least one volume of liquefied natural gas held in at least one storage tank, comprising compressing the vapour in first and second vapour compression stages in series, condensing the compressed vapour in a condenser by heat exchange with a working fluid flowing in a main endless refrigeration cycle, and returning at least some of the resulting condensate to the said storage tank, wherein in the main working fluid cycle the working fluid is, in sequence, compressed in at least one working fluid compressor, cooled in a first heat exchanger, expanded in an expansion turbine, employed in the condenser to perform the condensation of the natural gas vapour, warmed in the said first heat exchanger in heat exchange with the working fluid being cooled and returned to the said working fluid compressor, characterised in that in the main working fluid cycle intermediate the passage of the working fluid through the condenser and its passage through the first heat exchanger, the working fluid is employed to pre-cool in a second heat exchanger the compressed natural gas vapour downstream of the second vapour compression stage but upstream of the condenser, and in that a flow of working fluid is diverted from a region of the main working fluid cycle where the working fluid is flowing from the condenser to the second heat exchanger and is passed through at least one third heat exchanger so as to cool the natural gas vapour intermediate the first and second vapour compression stages, the diverted working fluid being returned to the main working fluid cycle at a region where the working fluid is flowing from the second heat exchanger to the first heat exchanger.
 2. The method according to claim 1, in which the proportion of the working fluid that is diverted from the main working fluid cycle to the third heat exchanger is controlled in response to the temperature at the inlet to the second vapour compression stage.
 3. The method according to claim 1, wherein when the said storage tank is fully laden with liquefied natural gas the condenser is operated such that sub-cooled liquefied natural gas exits from it.
 4. The method according to claim 1, wherein the vapour upstream of the first vapour compression stage is pre-cooled by having mixed therewith a stream of condensed natural gas taken from the condenser.
 5. The method according to claim 4, wherein the flow rate of the stream of condensed vapour is controlled in response to the temperature at the inlet to the first compression stage.
 6. An apparatus for reliquefying natural gas vapour comprising at least one storage tank for holding at least one volume of liquefied natural gas, first and second vapour compression stages in series for compressing boiled-off natural gas vapour communicating with at least one vapour space in the said storage tank, a condenser for condensing the compressed vapour having a natural gas inlet communicating with a second vapour compression stage and an outlet communicating with the said storage tank, wherein the condenser is arranged so as to be cooled, in use, by a working fluid, the condenser forming part of an endless main working fluid cycle comprising, in sequence, (a) at least one working fluid compressor for compressing a flow of the working fluid, (b) a cooling path through a first heat exchanger for cooling the working fluid flow, (c) an expansion turbine for expanding the flow of working fluid, (d) the condenser, (e) a warming path through the first heat exchanger for warming the working fluid, and (f) an inlet to the said working fluid compressor, characterised in that the main working fluid cycle comprises the second heat exchanger for cooling the natural gas by heat exchange with the working fluid, the second heat exchanger having a natural gas vapour path therethrough intermediate the second vapour compression stage and the condenser and a working fluid path therethrough intermediate the working fluid outlet from the condenser and the inlet to the warming path through the first heat exchanger, and in that there is a third heat exchanger for cooling the natural gas vapour intermediate the first and second natural gas vapour compression stages by heat exchange with working fluid diverted from the main working fluid cycle, the third heat exchanger having a working fluid path therethrough communicating at its inlet with a region of the working fluid cycle intermediate the working fluid outlet from the condenser and the working fluid inlet to the second heat exchanger and at its outlet with a region of the working fluid cycle intermediate the working fluid outlet from the second heat exchanger and the inlet to the warming path through the first heat exchanger.
 7. The apparatus according to claim 6, wherein there is a valve for controlling the proportion of the working fluid that is diverted from the main working fluid cycle to the third heat exchanger in response to the temperature at the inlet to the second vapour compression stage.
 8. The apparatus according to claim 6, wherein the first and second vapour compression stages are driven by a single plural speed motor.
 9. The apparatus according to claim 6, further comprising a mixer upstream of the first vapour compression stage in which the natural gas vapour is able to be cooled, the mixer having an inlet for condensed natural gas communicating with the condenser.
 10. The apparatus according to claim 9, further comprising a valve for controlling the flow of condensate to the mixer and operable to maintain constant the temperature at the inlet to the first compression stage.
 11. The apparatus according to claim 6, wherein an outlet for condensate from the condenser is able selectively to be placed through an expansion valve in communication with a phase separator having an outlet for returning liquid to the storage tank and an outlet for passing vapour to a combustion unit. 